I have been spotting (light flow and brown) for 2 week and then off 2 week. no period. i am on birth control, skip placebo. what could be the cause?

Thinning of the: uterine lining can be the most likely cause. The lining thins from the use of oral contraceptives, and usually is a good thing. If bleeding continues talk to your doctor, however taking a break in your pills for 5 days may allow the lining to thicken slightly and stop the bleeding. You would need to use condoms for intercourse, if using this idea, talk to your doctor first.